The present invention relates to an imaging apparatus, specially relates to an imaging apparatus for the reference mark measurement and inspecting the paste solder printing; more specially relates to an imaging apparatus for the reference mark measurement of the screen and the printed circuit board during the paste solder printing of the printed circuit board of the screen printing process, and for the inspection of the screen and the printed circuit board during the paste solder printing.
During the paste solder printing of the printed circuit board of the screen printing process, in order to apply the solder paste to the printed circuit board accurately, the portions of the printed circuit board desired to be printed are required to be corresponding to the holes of the screen. Reference mark measurement is usually used for the alignment of the screen and the printed circuit board, that is to say, signs are marked on both the screen and the printed circuit board so as to ensure that the portions of the printed circuit board desired to be printed are corresponding to the holes of the screen when the relevant signs of the screen are calibrated with respect to that of the printed circuit board. The simplest method of the alignment is mechanical positioning, in which the reference marks are the holes formed on the printed circuit board, and the sinker bars of the pistons of one or more cylinder-piston means are inserted into the positioning holes of the printed circuit board when positioning. However, this method is of the defects including low precision, low speed and even that there is no positioning hole due to the different positioning aperture of different printed circuit board. Therefore, this method is difficult to adapt to the development of the SMT production with high density, large output and high precision, and will fall into disuse gradually.
With the development of the electronic component in the direction of micromation, chip type and high density, the screen printer is required to be of higher precision, speed and reliability. Currently, in the SMT product line of the printed circuit board with high density, most of the defects come from the printing faults of the solder paste. The quality inspection of the solder-paste printing is mainly carried out by manual inspection. The manual inspection may have the problems including strong subjectivity, poor reproducibility, misjudgment and overlook due to vision fatigue, etc. The manual inspection is hardly qualified for the inspection requirement of SMT product line.
A screen printer utilizing machine vision technique can realize simultaneously both alignment with respect to the reference mark in high speed and high precision, and printing quality inspection during printing. At present, the imaging apparatus of the screen printer of this type normally includes a low-angle ring light source, two light sources, a beamsplitter, an optical reflector, an imaging lens and an image sensor. During the operation, the two light sources on different horizontal planes illuminate at different time, the light sources irradiate on the screen or the printed circuit board through the beamsplitter, and then the beams of light are reflected back to the beamsplitter and refracted to the imaging lens by the beamsplitter, and finally imaging on the image sensor is obtained. This method is of the advantages including simple light path and low cost. However, acquiring the images at different time may result in that the image acquiring positions of the lower image and the upper image are inconsistent; and the dimension of the imaging apparatus in the direction of the upward and downward light path is too large, so that the distance between the printed circuit board and the screen is accordingly large when acquiring the image.